Diesel-type compression ignition (CI) internal combustion engines achieve combustion by compressing a charge of air in the firing cylinder and injecting one or more portions of fuel into the compressed air, creating a fuel/air mixture that is spontaneously combustible at a threshold combination of temperature and cylinder pressure. The apparatus, methods, and timing of the fuel injections to maximize fuel efficiency and engine performance and to minimize exhaust pollutants are the subjects of a highly-developed engineering prior art.
In early diesel art, fuel injection was carried out as a single injection at or near the top dead center (TDC) of the compression stroke of the engine piston. More recently, it has been recognized that improved fuel economy and engine efficiency, and reduced pollution, can be achieved by injecting fuel multiple times during the compression stroke to provide a more uniformly dispersed and evaporated fuel/air charge that ideally will detonate uniformly and simultaneously throughout the firing chamber, rather than simply combusting in a lower-energy progressive wave from an initial site.
It has further been found that timing of the onset of fuel injection is a critical factor in achieving these goals. Implementation of real-time combustion feedback for use in closed-loop combustion control is a technology that has potential to assist in the successful production implementation of advanced diesel combustion modes.
Low-temperature, pre-mixed diesel combustion is of interest because it offers the ability to lower the engine-out emissions of oxides of nitrogen (NOx) and particulate matter (PM) such as carbon soot. The need for lowering these two emissions is driven by tighter regulations enacted worldwide, especially concerning NOx limits. Reducing engine-out emissions eases the need for additional exhaust aftertreatment devices and their associated cost and mass.
Low-temperature, pre-mixed combustion in diesel engines consists of injecting the fuel such that it is allowed to vaporize and mix with the intake mixture before combustion occurs. A high level of exhaust gas recirculation (EGR) is often used to reduce the combustion temperature. Since all of the fuel typically is injected prior to the start of combustion, the crank angle when combustion starts is controlled by the chemical reaction kinetics of the mixture, which are directly influenced by the pressure and temperature of the mixture, among a number of other factors. This introduces new variability factors that are not present in traditional diffusion-burn diesel combustion wherein start of combustion occurs at a cetane-number-based time delay after the start of fuel injection.
While pre-mixed combustion is desirable for emissions reduction, a challenge is in maintaining the stability of combustion during transient events and transitions between combustion modes. Closed-loop feedback control of fuel injection timing offers the potential to improve controllability of pre-mix combustion, thus enabling a means of improving engine-out exhaust emissions.
A known approach for controlling fuel injection timing is disclosed in SAE Technical Paper No. 2007-01-0773, the relevant disclosure of which is herein incorporated by reference, wherein in-cylinder combustion pressure is used for providing combustion feedback. When sampled real-time with the appropriate electronics, the resulting cylinder pressure waveform can be analyzed to determine combustion parameters such as the angle of start of combustion; indicated mean effective pressure (IMEP); angular location corresponding to when 50% of the total heat release has occurred; and the angular location of peak pressure (LPP). Such combustion feedback can then be used to adjust relevant engine control parameters, including timing of the start of fuel injection. By having a pressure sensor in each cylinder of a multi-cylinder engine, combustion feedback allows for combustion phasing on an individual cylinder basis and a resulting ability to correct for cylinder-to-cylinder variations as well as to follow a commanded target value.
A shortcoming of the disclosed system is that it requires substantial and sophisticated hardware, including electronic pressure sensing means in each cylinder, which is not readily available in either sufficiently-rugged form to permit widespread adoption or at a reasonable unit cost.
What is needed in the art is a simplified means for controlling fuel injection timing to improve engine performance, especially in mass-produced road-use vehicle engines, and especially during transient periods of engine operation.
It is a principal object of the present invention to improve CI engine performance and reduce NOx and PT emissions during transient periods of engine operation.